For more than a century, Turkey has denied any role in organizing the killing of Armenians in what historians have long accepted as a genocide that started in 1915, as World War I spread across continents. The Turkish narrative of denial has hinged on the argument that the original documents from postwar military tribunals that convicted the genocide’s planners were nowhere to be found.

Now, Taner Akcam, a Turkish historian at Clark University in Worcester, Mass., who has studied the genocide for decades by piecing together documents from around the world to establish state complicity in the killings, says he has unearthed an original telegram from the trials, in an archive held by the Armenian Patriarchate of Jerusalem.

“Until recently, the smoking gun was missing,” Mr. Akcam said. “This is the smoking gun.” He called his find “an earthquake in our field,” and said he hoped it would remove “the last brick in the denialist wall.”

The story begins in 1915 in an office in the Turkish city of Erzurum, when a high-level official of the Ottoman Empire punched out a telegram in secret code to a colleague in the field, asking for details about the deportations and killings of Armenians in eastern Anatolia, the easternmost part of contemporary Turkey.

Later, a deciphered copy of the telegram helped convict the official, Behaeddin Shakir, for planning what scholars have long acknowledged and Turkey has long denied: the organized killing of up to 1.5 million Armenians by the leaders of the collapsing Ottoman Empire, an atrocity widely recognized as the 20th century’s first genocide.

And then, just like that, most of the original documents and sworn testimony from the trials vanished, leaving researchers to rely mostly on summaries from the official Ottoman newspaper.

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Instead, he found a photographic record of the Jerusalem archive in New York, held by the nephew of a Armenian monk, now dead, who was a survivor of the genocide.

While researching the genocide in Cairo in the 1940s, the monk, Krikor Guerguerian, met a former Ottoman judge who had presided over the postwar trials. The judge told him that many of the boxes of case files had wound up in Jerusalem, so Mr. Guerguerian went there and took pictures of everything.

The telegram was written under Ottoman letterhead and coded in Arabic lettering; four-digit numbers denoted words. When Mr. Akcam compared it with the known Ottoman Interior Ministry codes from the time, found in an official archive in Istanbul, he found a match, raising the likelihood that many other telegrams used in the postwar trials could one day be verified in the same way.

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The genocide is commemorated each year on April 24, the day in 1915 that a group of Armenian notables from Istanbul were rounded up and deported.

It was the start of the enormous killing operation, which involved forced marches into the Syrian desert, summary executions and rapes.

Two years ago, Pope Francis referred to the killings as a genocide and faced a storm of criticism from within Turkey. Many countries, including France, Germany and Greece, have recognized the genocide, each time provoking diplomatic showdowns with Turkey.

It is well past time for the US to recognize the genocide.

In fact, I think that if anything, we need to be even more aggressive about this than we are about Holocaust deniers, because they are (thankfully) a relatively small fringe group, while the Turkish state continues to continue this crap in their mandatory education curriculum, polluting the minds of the next generation.